There's a couple of things that can happen here to cause this error.
The first, and probably more common now is that the client's authentication times out. Authentication is valid for 24 hours only. This is a limitation Mojang put on Minecraft accounts ages ago. It's a security feature, annoying as it is. There's nothing Minecats can do for this. There's a couple of solutions you can attempt however: close Minecraft and go back into the launcher. If your launcher closes when Minecraft opens, you're done. If you keep your launcher open after Minecraft starts, you can either close the launcher and re-open, -or- in the top right corner click on your IGN, choose "switch account" and click on your IGN again - this forces a Minecraft to re-authenticate with Mojang. Now you can re-open Minecraft and connect to Minecats.
[EDIT: Additional reason added]
When a player changes their IGN, the above authentication issue, regardless of how old the authentication token is, is nullified immediately. Thus, you must still log out of the client and launcher and re-open the launcher to re-authenticate with Mojang's servers.
The second issue is not something Minecats or you can solve. This happens when Mojang's Authentication servers go offline. There's a few reasons for this, but generally it does not happen any more. But it can. The only solution is to wait until those servers come back online.
You can ask in Discord if others are experiencing the issue as well. If they are, then the servers are down - there are websites where you can see the current status of those servers. If no one else is having issues, you may need to restart your PC completely. If that still don't help, then something else is wrong and you should probably find someone who can walk you through diagnosing the problem and help fix it.
tl;dr:
close Minecraft, re-open or "switch account" to re-authenticate with Mojang. If that don't work, ask others in Discord if they're having the same issue. If they are, you will have to wait. If they are not, then try again. If you still can't, then seek competent assistance.
